Worn Carb Heat Arm....

Ironflight · #1 05-21-2007, 08:07 PM

When I got back from Carlsbad yesterday, I thought something looked funny up front - sure enough, my carb heat door was hanging down vertically in front of the air filter, blocking off the intake. It was hanging loose, and was pretty much acting like a blow-in door - I'm sure it was fully horizontal by the time I had flying speed! (Hey, blow-in doors to prevent FOD and dirt on the ground - very high tech!) Clearly, it was disconnected from the cable end, or the cable was broken.

I went out tonight to investigate, pulled the cowl, and the problem was obvious - the "bug nut" had enlarged the hole in the end of the door's control arm and pulled through. Don't ask me what happened to the washer and cotter pin - they were gone! Don't know that could happen.....

The proper fix is probably to replace the door, hinge, and control arm completely, as the hinge is getting pretty "loose" as well, but I figure that if it lasted 475 hours, it will probably make it to the next condition inspection, so I simply drilled a new hole or the bug nut a little closer to the pivot point, installed a new washer and cotter pin, and called it good to go.

The carb heat door, hinge, and lever are high wear points in many of the RV's I have seen (with the stock config), so I am not surprised at this. As we all know, there isn't much in the way of carb heat anyway...but it's nice to have the pieces that we have all where they are supposed to be, and operating properly....

Once again, the Val does accelerated life testing for the fleet....475 hours in 19 months!

Paul

Bob Axsom · #2 05-21-2007, 08:56 PM

I found the wear problem on my plane and added a rod end bearing with stud to the end of the control wire. It was posted somewhere here with a photo.

Bob Axsom

Radomir · #3 05-21-2007, 10:04 PM

Paul, just curious... is the control arm made of steel or aluminum?

Ironflight · #4 05-21-2007, 10:33 PM

Quote:

Originally Posted by Radomir

Paul, just curious... is the control arm made of steel or aluminum?

Steel....but so is the "bug nut"....

Bob has a good idea - I'll have to rememebr that when I re-design!

Paul

AlexPeterson · #5 05-22-2007, 06:51 AM

Paul,

I also had wear in that hole (one hates to think of the vibration necessary...). I fixed it by sliding a piece of reinforced silicone tubing over the arm (causing the tube to go oval or flat). I had punched a hole through both sides of the tube in the appropriate location, and put the bolt through the tube. My setup has a drilled bolt holding a push cable. This would require the basic metal arm to still have an intact, albeit enlarged, hole. The hole in the tubing was sized to still allow rotation of the bolt, but to not allow the bolt to really contact the metal arm. Sort of like car suspension joints.

gmcjetpilot · #6 05-22-2007, 07:59 AM

Not sure I follow all of your issue, but assuming its a FAB (Filtered Air Box) Van.

-Drill out old hinge and replace with an extruded hinge, not the cheap rolled hinge that comes with the kit, but use the "MS 20001P"
http://www.aircraftspruce.com/catalo...pianohinge.php

-Replace carb heat cable with one with solid threaded end, not wire.
http://www.aircraftspruce.com/catalog/appages/a770.php

-Use a clevis or mini mono ball (bearing) fitting to attach to arm. The threaded control end screws into that.
http://www.vansaircraft.com/cgi-bin/...ct=re-bearings

The hinge will not wear out. The cable will not break and you will have positive opening and closing. It's not cheap but you will never have to fix it, at least for a long time.

Scorch · #7 05-22-2007, 05:21 PM

Iron,

I feel your pain. I'm just back from the airport after yet another airbox repair. Mine breaks in the area around the hinge - today the flapper valve was basically hanging by the actuating cable. I've made several repairs to this area in 685 total hours engine and airplane. Watched in awe today as Stan Price "helped" me fabricate a bracket to hold it all together.

I'm thinking the design of this airbox was maybe not Van's finest hour. Took my repaired airbox to Randy "Monkey" Richmond to have it blessed before I reinstalled it - he tells me there are lots of cracks and breaks in this hinge area on RVs in general, caused by vibration. He was hard at work on Larry Pardue's airplane, and he showed me on Larry's engine an idea he had.

The crossover exhaust runs horizontally directly over the forward part of the airbox. You could hang a couple of exhaust hangers on the pipes, then run brackets from them to the forward sides of the airbox to reduce the vibration. Monk says the vibration is at it's worst on engine start and shutdown.

Anyway something to think about - if my airbox keeps breaking I'll try Monk's idea.

See ya at TEX this Saturday...

Ironflight · #8 05-22-2007, 07:53 PM

I figure that at 475 hours, I have to expect a little wear from vibration, so I guess I'm doign better than many!

Paul

Sheldon · #9 05-23-2007, 06:14 AM

I had wear with the hinge causing the the flap to become loose and floppy at about 325 hours.
I replaced the hinge with the next larger size, inserted a small teflon tube into the hinge eyes... Then the pin (from the smaller hinge size) into the tube... Making for a kind of bearing for the hinge/pin... It all fit very well...Worked perfectly and at now 450 hours, doing fine.. Also have a bearing at the end of the cable to the control arm. There has been no wear there...

gasman · #10 05-23-2007, 07:11 PM

Quote:

Originally Posted by Ironflight

As we all know, there isn't much in the way of carb heat anyway...but it's nice to have the pieces that we have all where they are supposed to be, and operating properly....

Once again, the Val does accelerated life testing for the fleet....475 hours in 19 months!

Paul

Can someone with carb temp tell us what temp. increase you get. I can only see about a 20 rpm drop with carb heat. Is it enough?